Brushless DC motors, as they are called, are driven directly from a polyphase AC supply, which energizes a system of stator windings. The stator assembly, in response to the phase drive currents, produces a rotating magnetic flux, which interacts with the rotor flux. This in turn applies torque to the rotor, attempting to align its flux with the rotating field. Thus the rotor turns in synchronism with the stator flux and at a precise multiple (or more correctly sub-multiple) of the supply frequency. In modern motor drive systems the brushless motor is driven from a mains supply via an electronic power converter unit. This produces both a variable frequency and variable amplitude phase drive for the motor to allow control of motor speed over a range of load torque conditions.
Distributed about the rotor assembly of these machines is a system of permanent magnets that are arranged to provide a radial flux distribution mirroring the flux distribution of the rotor magnetic field. FIG. 1 of the accompanying drawings is a diagrammatic sectional view through a typical rotor construction for a 3-phase 2-pole-pair machine. Here, a system of four radially polarized magnets is disposed about the surface of a shaft to give a symmetrical flux pattern of two pole-pairs (north-south pairs). In this arrangement, the shaft acts both as a support for the rotor and as a low reluctance return circuit for the flux entering or leaving the inner faces of the magnets.
Elimination of the commutator, resulting in the brushless dc motor, has brought about a number of benefits these including freedom from arcing and commutator erosion, and a considerably more compact, simplified and rugged assembly. This in turn has led to the development of very high performance machines where operating speed and overall power density is greatly increased over the earlier mechanically commutated designs. At elevated operating speeds the design of the rotor assembly however becomes increasingly critical. Considerable mechanical stability and accuracy of form of the assembly is required to minimize destructive imbalance forces, and the structure must be designed to withstand the centrifugal or bursting stresses induced by rotation.
At high operating speeds the prior art rotor design of FIG. 1 shows limitations. The alloys or compositions of modern high performance magnets are brittle materials, exhibiting relatively low strength. They are not capable of withstanding significant tensile stress and thus require confinement to protect them from disintegrating. The prior art rotor design of FIG. 1 is generally achieved using a prior art overall constraining sleeve manufactured from a material such as filament wound carbon fiber that is not permeable to the rotor flux and that exhibits high tensile strength. Use of the prior art overall constraining sleeve however brings with it the disadvantages of an increase in the stator-rotor gap across which flux must be driven, and a corresponding reduction of the structural core diameter of the rotor. For the same motor efficiency, a greater volume of magnet is now required which results in a further reduction in rotor core diameter.
For high-speed rotation, the bending stiffness of the shaft is an important consideration. The self-mass or density and the body stiffness of the rotor will determine the frequencies of a series of modes of vibration. If the motor dwells, even for short periods of time, at a speed corresponding to the frequency of one of these modes, considerable energy can be coupled into transverse vibration and this can cause bearing damage or catastrophic failure of the shaft. In the design of these high-speed machines therefore, considerable effort is invested in achieving stiff and lightweight structures, while simultaneously providing constraint for the magnet assembly to eliminate or minimize tensile stresses within the material. Since the shaft stiffness in bending is proportional to the fourth power of the structural core diameter, it can be appreciated that in this prior art design, a careful trade off must be struck in the allocation of materials to the structural core, magnet and prior art constraining sleeve components of the rotor.
Accordingly, what is needed is to overcome the shortcoming and the drawbacks of the prior art, and to provide a rotor construction which allows high shaft stiffness together with the achievement of good magnet constraint conditions. The invention further offers the benefits of an extremely simple construction with no redundant magnet confining material.